Pneumatic transportation is used extensively in process industries. In some industries such as tobacco and food processing changes in the physical particle size distribution and product moisture are detrimental to final product quality and manufacturing economics. Impaction and abrasion of the product onto itself and onto the transport duct walls reduces the particle size and generates dust. If these reduced size components are incorporated into the final product, product quality is reduced and consumer satisfaction placed at risk. Their removal from the product increases wastage and material cost, or if a recovery process exists, they then incur additional recovery and conversion costs.
Generally the higher the air/gas transport duct velocity, the greater is the risk of such size degradation.
Considering those transport ducts used in the tobacco industry to supply material to cigarette makers, the air/gas velocity is typically 17 to 21 meters per second. It is recognised that the lower velocity causes much less product damage than does the higher velocity.
It is recognised that different stages within the transport duct require different energy inputs to the product via the air velocity. For example more energy is required to transport material horizontally than vertically due to the tendency of the material to settle out in a horizontal duct. Also transport round a bend requires more energy per unit length of duct than transport in a straight line.
Ignoring leakage and compression effects the volume of air/gas flowing along a section of a transport duct is substantially constant. This volume is set sufficiently high to meet the maximum local energy level requirement and energy losses due to length of duct and number or rises and bends. It is becoming more common to alter the size of sections of duct to give a local increase or decrease in velocity. For example, horizontal and bend sections could be of a reduced diameter, while declined sections could be of larger diameter. However, this approach can cause additional problems at the transition between sections.
Using cut tobacco as an example, the entrainment velocity is typically around 2 meters/second, compared to the 17 to 21 m/s ambient air velocity commonly used. The velocity of material movement in the direction of air movement is the difference between the entrainment velocity and the air velocity.
Consequently if air containing tobacco is moving in a horizontal duct as 17 m/s then the tobacco will be moving horizontally at about 15 m/s. However, gravity still acts and tobacco in the duct will move downwards at about 2 m/s. When this tobacco touches the duct wall it will slow down and may come to rest temporarily. As the depth of slow moving tobacco builds up it restricts the duct causing an increase in air velocity over the restriction. When the air velocity is sufficiently high the tobacco causing the restriction will be re-entrained into the main airstream.